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Monday, July 07, 2008

Recreating the Big Bang?

With the start of the Large Hadron Collider coming closer, the topic is present in the media more than ever. A commonly used motivation is the alleged recreation of the Big Bang (see illustration to the right).Peter Woit recently mentioned that Martinus Veltman, winner of the '99 Nobelprize in physics, “described claims that the LHC will 'recreate the Big Bang' as 'idiotic', and as 'crap'. He said that this is 'not science', but 'blather', and that the field would come to regret this, arguing that if you start selling the LHC with pseudo-science, you will end up paying for it.”

I am totally with Veltman. But what is behind the story? What does the LHC have to do with the Big Bang?The Making Of
It is interesting to trace back how this inaccurate description developed. In February 2000, BBC News wrote on CERN's SPS:

Smashing together atoms to produce conditions similar to those in the first cosmic moments, scientists came up with some startling results that could force them to reexamine their understanding of the universe.

It is a $4 billion instrument that scientists at the European Center of Nuclear Research, or CERN, hope to use to re-create the big bang — believed to be the event that caused the beginning of the universe — by crashing protons together at high speed.

True, the LHC speeds up particles to higher energies than SPS, but still this is far off from anything similar to the Big Bang.

The Big Bang

The Big Bang is believed to be the first moment of the universe. Technically seen, it takes place at arbitrarily high energy density. It is commonly expected however that in this regime quantum gravity becomes important, and the density is neither infinitely high nor is the volume arbitrarily small. But still the temperatures for this to happen would be somewhere in the Planckian regime, that is at average energies of about 1016 TeV.

To our best current understanding, the universe then undergoes a rapid phase of expansion during which all energy densities drop and all matter cools. With dropping temperature, we pass the scale above which we expect Grand Unification and the three forces of the standard model separate. This is believed to be somewhere at 1013 TeV. Then around a TeV there is the electroweak phase transition. At some hundred MeV, that is about 10-4TeV, quarks start to form bound states like protons and neutrons. This is commonly called hadronization. It is this transition that we can now hope to study in appropriately designed collider experiments [2].

After hadronization, at temperatures around one MeV (10-6TeV), atomic nuclei can form - a process that is called 'nucleosynthesis'. Around this temperature also the only weakly interacting neutrinos decouple [3]. At temperatures of the oder eV (10-12TeV) atoms form and photons decouple. These photons have been traveling freely since this so called `freeze-out'. We can observe them today in the cosmic microwave background with an average temperature of around 3K (10-3eV) because they have been further redshifted by a factor 1000 since the freeze-out. After freeze-out, structure formation sets in, first stars, galaxies, solar systems and planets form. Some of these planets might carry intelligent life, some might even have a blogosphere.

There are several important differences between the conditions created at the LHC and the Big Bang.

The LHC main program is proton-proton collisions. There is no sensible way in which one could understand the conditions created in these particle collisions as a thermal density distribution. These are scattering experiments. (Though some of the data obtained in these experiments can have thermal characteristics, this does not mean it was indeed similar to the early universe.) The LHC will also have a heavy ion program in which lead nuclei are collided which each other. In these circumstances it is more appropriate to speak of actually creating an intermediate state with a high density and energy density.

However, in such heavy ion collisions, the produced state of high density from the two nuclei expands much more rapidly than would be the case in the early universe. Everything is over within the time span needed for light to cross a few diameters of the colliding lead nuclei, or a few 10-22 seconds. In fact, the expansion is so rapid that it is not even clear from the outset if one can expect any thermalization. In contrast to this, in the early universe the hadronization transition happens after about the first microsecond, and the Hubble expansion is so slow compared to the back and forth of the quarks and gluons that it's granted the early universe is thermal. (Again, though some of the data obtained in heavy ion experiments has thermal characteristics, this does not mean it was indeed similar to the early universe.)

Also, in the early universe the expansion of the matter is due to the expansion of space itself. In the laboratory, it is the matter that expands in an to very good approximation flat and static background. Though this might not make a difference for the cooling of the matter, it is conceptually very different.

The typical temperature that is created in heavy ion collisions is some hundred MeV. That is about 19 orders of magnitude below the temperature we expect at the Big Bang.

Bottomline

The LHC is not a Big Bang machine. It is more accurate to say that with the heavy ion program at the LHC we will be able to create conditions closer to that in the early universe than ever before. This sounds more boring, but at least it isn't blatantly wrong. Aside from this, it is more useful to think of the LHC it as the world's largest microscope, that will help us to peer into the structure of elementary matter to a resolution better than ever before.

24 comments:

Hi Bee and Stefan,I think there is a very simple explanation for this confusion, as somone who dabbles a little in science journalism.

The problem is that scientists frequently make statements like "such and such an accelerator will re-create energies not seen since the Big Bang". Not technically incorrect, but misleading..

It is not made clear to journalists that what is really meant is "energies not seen since shortly after the BB" (a different thing you'll agree) - so they therefore assume we are re-creating the Bang itself!

Nice piece, this recreating the early universe stuff is obviously what makes for good headlines and was never intended to be good science. Also it is probably considered by even those involved as what’s required in selling the public on the exspendager being justified. To describe it as simply a better microscope just isn’t sexy enough I’m afraid for those that have no idea about the frontiers of physics. On the other hand, it has had its backlash with many believing the scientists are playing god or some such silliness. This has a lot in common with the doomsday scenarios that proved to be so hard to dispel.

Just as another side remark, which is how can one logically justify statements like ‘exploring energies not seen since the beginning of the universe’ with ‘not worrying about the energies created for they are far exceeded by everyday cosmic ray events’. Like the old saying goes, ”one cannot suck and blow at the same time”:-)

I understand that it must be difficult to be a science journalist and to find the right amount of details that can sensibly be communicated in popular writing. But I am really tired of hearing excuses like this. If a journalist doesn't know what energies 'not seen since the Big Bang' means, he or she should ask and clarify and not fantasize. Besides this, it isn't hard to find out. If you look at the three examples I have in the post, you will see that the first one is actually quite accurate, but then the explanation became increasingly fuzzy and eventually outright wrong. Every decent science journalist should have been able to figure that out. I suspect that there is just little motivation to actually get things right, but instead there is an emphasize on advertising and entertaining, and the more bang the better. Best,

"I suspect that there is just little motivation to actually get things right, but instead there is an emphasize on advertising and entertaining, and the more bang the better."

It's about making money. The exact reason why so-called cosmological "implications" of high energy physics research are eagerly hyped by the media was explained by Jeremy Webb, former BBC sound engineer and now Editor of New Scientist, in Roger Highfield's article "So good she could have won twice" in the Daily Telegraph, 24 Aug. 2005:

Since Jeremy's job includes the requirement to make sure that the magazine sells well, it's obvious why any alleged development in particle physics which has a cosmological connection is likely to end up as the story spin on the front cover.

The easiest way for a science journalist to begin an article about the LHC in a way that will grab an editor's attention and the typical reader's attention, is to write something like:

"The purpose of the LHC is to recreate exactly the conditions which occurred in the big bang, allowing us to see precisely what occurred during the creation of the universe."

I'm glad that Martinus Veltman and bloggers such as Peter Woit and yourselves decided to be honest about what the LHC really is up to. It's interesting enough to investigate the electroweak symmetry breaking mechanism, without the really interesting physics of the LHC being misrepresented by the BBC News and other purveyors of nonsense.

Plato: It would really help if you could be more precise, I neither have the time nor the patience to guess around. This picture you link to with an incredibly bad resolution could

a) Show an incoming cosmic ray that creates a shower by scattering on something in the Earth's atmosphere. It neither says what the incoming particle is in this case, nor what it hits in the primary collision, nor what the energy is you are looking at. It is commonly believed that cosmic rays are caused by protons, but they hit atoms in the atmosphere, which contain, but are not just protons. The energy scales of cosmic ray collisions range from energies far below to somewhat above what the LHC can reach, the signatures are very different because the created particles don't just hit a detector in some meters distances but they travel some more thousand meters through the Earth's atmosphere.

b) Show a proton scattering on the CMB background. The LHC doesn't scatter protons on photons.

Layman apologies:) You don't have to answer any more if you don't like too.:)

Cosmic rays are caused by protons from outer space. When a proton (shown in yellow) hits the air in the earth's upper atmosphere it produces many particles. Most of these decay or are absorbed in the atmosphere. One type of particle, called muons (shown in red), lives long enough that some reach the earth's surfaceSee: Introduction to Cosmic Rays

You don't have to apologize for being a layman, but I can't answer questions if I don't know what the question is. Yes, the LHC will produce muons. See these funny looking caps of ATLAS? They are part of the muon detector. Best,

The Pierre Auger Observatory in Malargue, Argentina, is a multinational collaboration of physicists trying to detect powerful cosmic rays from outer space. The energy of the particles here is above 1019eV, or over a million times more powerful than the most energetic particles in any human-made accelerator. No-one knows where these rays come from.

This is why John Ellis was instrumental in my comprehension of what is taking place.

To deny astrophysics in correlation to the work being done in the LHC is how you might say "a nutty response" to what is actually happening in reality.:)

Hey Bee,I'm not a science journalist either!Re making the minimum effort at understanding, I think you're broadly right, but have defnitely come across this issue as a genuine misunderstanding more than once.

I guess like most teachers, I tend to blame myself if those I'm trying to inform misunderstand - it's easy to make statements that are ambiguous / misleading.

It's certainly an important issue - I suspect the confusion of LHC energy with BB is partly reponsible for the BH scaremongering..Cormac

The following link:http://arxiv.org/abs/astro-ph/9903300on pg. 11 has an interesting figure representing the BBNS time/temperature evolution for light nuclei. The highesttemperature is roughly equivalent to 10 MeV (LHS of the figure) . Question: you see anything wrong in extrapolating the p,n,d and A=3 curves to 150 MeV? The physics shouldn't change much. The nucleon-nucleon x-section decreases with energy, but the baryonic density increases, so the (say) deuteron fraction actually increases with temperature. Is this ok?

... well, at around 150 MeV at latest, the quark-gluon substructure comes into play, but even before, there should be thermal pions around in non-negligible numbers.

I really doubt how bound nuclei can be stable in these conditions - especially deuterons, with a binding energy of only about 2 MeV (1 MeV per nucleon). But even for He-4, binding energy per nucleon is about 7 MeV, so I am not sure if one can do this extrapolation.

Dear Stefan,At the T = 10 MeV limit (LHS of the referred BBNS figure) photon disintegration is said to be responsible for the small amount of d, and A=3 nuclei. My argument is that their abundance IS small, but definitively non-zero, and seems to increase with energy. The pion-pair production channel opens up at E = 240 MeV, although (sure) at T = 150 MeV the thermal-tail contribution should be important. My point is: although individual light nuclei would (indeed) have a small survival rate, their overall abundance at those temperatures would not be zero, and may even increase with energy. Can you refer me to someone working in this field today. I wrote to those authors but haven't heard from them, yet. Thanks for your prompt response.

My argument is that their abundance IS small, but definitively non-zero, and seems to increase with energy. ... My point is: although individual light nuclei would (indeed) have a small survival rate, their overall abundance at those temperatures would not be zero, and may even increase with energy.

True, besides just the binding energy, there are other factors in play - most importantly, I would guess, the energy-dependence of the cross sections and density in phase space. After all, there should be an equilibrium state changing adiabatically with time/dropping temperature (if I recall correctly, Hubble rate at this period is already much lower than typical collision/reaction rates...)

Now, I am not an expert at all at primordial nucleosynthesis, but naively, I would not expect the fraction of bound nuclei to increase very much further with temperature/energy.

Sorry, I haven't worked on this topic and don't know anyone whom I could ask directly. Maybe you can try to mail some textbook authors who show the plot, or the old-fashioned sci.phys.research, or a physics forum?